ACOUSTICS Aural Comfort & Noise

Noise It affects our quality of life May cause distractions Mask wanted sounds Cause hearing loss

Sources of noise When is a sound a noise? Mechanical Equipment Transport Mechanical Equipment Community noise People When is a sound a noise?

Sound: “Is a form of vibrational energy which can pass through air, liquids and solids” Noise (legal definition): “Sound that is unwanted by the recipient.”

Continuous Noise Continuous noise is produced by machinery that operates without interruption in the same mode, for example, blowers, pumps and processing equipment. Measuring for just a few minutes with hand-held equipment is sufficient to determine the noise level. If tones or low frequencies are heard, the frequency spectrum can be measured for documentation and further analysis

Intermittent Noise When machinery operates in cycles, or when single vehicles or aeroplanes pass by, the noise level increases and decreases rapidly. For each cycle of a machinery noise source, the noise level can be measured just as for continuous noise. However, the cycle duration must be noted. A single passing vehicle or aircraft is called an event

Impulsive Noise The noise from impacts or explosions, e.g., from a pile driver, punch press or gunshot, is called impulsive noise. It is brief and abrupt, and its startling effect causes greater annoyance than would be expected from a simple measurement of sound pressure level.

Tones in Noise Annoying tones are created in two ways: Machinery with rotating parts such as motors, gearboxes, fans and pumps often create tones. Unbalance or repeated impacts cause vibration that, transmitted through surfaces into the air, can be heard as tones. Pulsating flows of liquids or gases can also create tones, caused by combustion processes or flow restrictions. Tones can be identified subjectively by listening, or objectively using frequency analysis

Low Frequency Noise Low frequency noise has significant acoustic energy in the frequency range 8 to 100 Hz. Noise of this kind is typical for large diesel engines in trains, ships, and power plants and, since the noise is hard to muffle and spreads easily in all directions, it can be heard for miles.

Sources of Noise Around a Building

Building noise

Transmission of airborne noise through a building. Transmission of impact noise through a building.

Subjective view of noise nuisance will depend on: Activity and environment Duration and repetition Type of noise The personal prejudices of the listener.

Physiological Effects Noise can affect humans in many different forms: loud noise (NIHL) vibrations (nausea) white finger disease

Time element of noise measurement Short duration noise will be more annoying than noise of a longer duration. One way of accounting for this is to use a percentile index. One example is the traffic noise index L10,18 The measurements use a weighted scale to take into account the range of frequencies being measured. The most common is the A scale.

Exposure limits Use is made of Equivalent Continuous Sound Level, LEQ LEQ,8 of 90dB(A) gives: 90dB(A) for 8 hours or 93dB(A) for 4 hours or 96dB(A) for 2 hours or 99dB(A) for 1 hour

Acoustics is the study of the behaviour of sound in an enclosed space. We judge the acoustic quality of a room by considering: Sound level Audibility Reverberation Ambient noise Echoes and or quality of sound Use of the room Auditorium are generally recognised as requiring good acoustics satisfying subjective criteria such as richness of tone, definition, blend and balance of sounds. However acoustic quality will be significant in most buildings.

We study room acoustics by a geometric study of the sound path in a space. Reflection, absorption, transmission and diffraction are involved.

Reflection being used to aid distribution of speech Sound path mechanisms Reflection being used to aid distribution of speech

Reflections can result in unwanted echoes Reflections can result in unwanted echoes. An echo will be heard if the time difference between hearing the direct sound and the reflected sound is 50ms or greater then we will perceive this as an echo. Room geometry may also result in standing or resonant sound waves. Non-uniform sound levels throughout a room will be a problem.

The rate of decay of reverberation has an impact on sound quality The rate of decay of reverberation has an impact on sound quality. The reverberation time is defined as the time taken for a sound to decay by 60dB. Speech 0.5 to 1 s Music 1-2s The time will depend on sound frequency, room dimensions, room surfaces - materials and areas

Optimum room times can be calculated using Stephens and Bates formula: The calculated time can be compared with published recommendations.

We can apply one of two simple formulae to predict the reverberation time of a space Sabine’s formula, used when absorption is not high and decay is continuous

Sound absorption is defined as the reduction in sound energy reflected from a surface. The absorption coefficient is the ratio of the absorbed sound energy to the incident sound energy.

We can combine our surface absorption information with that for the contents of a space to decide if it is necessary to adjust the absorption qualities of the space. In addition to use of say soft finishes, such as curtains, specific absorbers can be installed.